12 research outputs found
Fate of nitrogen atoms in the photocatalytic degradation of industrial (congo red) and alimentary (amaranth) azo dyes. Evidence for mineralization into gaseous dinitrogen
The photocatalytic degradation of two azo-dyesâan industrial one (Congo Red (CR)), and an
alimentary one (Amaranth (AM))âhas been investigated in TiO2/UV aqueous suspensions. In addition to a
prompt removal of the colors, TiO2/UV-based photocatalysis was simultaneously able to fully oxidize the
dyes, with a complete mineralization of organic carbon into CO2. In particular, the aromatic rings were submitted
to successive attacks by photogenerated OHâ radicals leading to hydroxylated metabolites before the
ring opening and the final evolution of CO2 induced by repeated subsequent âphoto-Kolbeâ reactions with
carboxylic intermediates. Simultaneously, sulfur heteroatoms were converted into innocuous SO42â ions. The mineralization of nitrogen was more complex to analyze. Nitrogen atoms in the -3 oxidation state, such
as in the amino-groups of CR, initially remained at this reduction degree and produced NH4+ cations, subsequently and very slowly converted into NO3â anions. For both azo-dyes (CR and AM) degradation, the overall
mass balance in nitrogen was always found incomplete. Various experiments performed in pure oxygen in
a vacuum-tight cell and then in an air-free photoreactor (but filled with pure oxygen) enabled us to put in
evidence the formation of N2. Quantitative measurements clearly indicated that gaseous dinitrogen evolved
stoichiometrically corresponded to the mineralization of the central âN=Nâ azo-group. This constitutes the
ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis
but also for any other physicochemical method. These results suggest that TiO2/UV photocatalysis may be
envisaged as a method for treatment of diluted colored waste waters not only for decolorization but also for
total detoxification, in particular in textile industries in semi-arid countries
Environmental green chemistry as defined by photocatalysis
International audiencePhotocatalysis is efficient in several fields. Firstly, in selective mild oxidation: oxidation of gas and liquid hydrocarbons (alkanes, alkenes, cyclo-alkanes, aromatics) into aldehydes and ketons. Primary and secondary alcohols are also oxidized into their corresponding aldehydes or ketones. The high selectivity was ascribed to a photoactive neutral, atomic oxygen species. Once platinized (only 0.5 wt.% Pt) titania may catalyze reactions involving hydrogen (deuterium-alkane isotopic exchange and alcohol dehydrogenation). For fine chemicals, high initial selectivities enable titania to address most of the twelve principles of âgreen chemistryâ, such as the synthesis of 4-tert-butyl-benzaldehyde, an important intermediate in perfume industry by direct selective oxidation of 4-tert-butyl-toluene with air. A new field recently appeared: thio-photocatalysis. Oxygen was replaced by sulfur, using H2S as a convenient and reactive source. For instance, the conversion of propene in 1-propanthiol was successfully obtained. The reaction was performed using either CdS or TiO2. The latter was much more active than CdS. In environmental photocatalysis, titania becomes a total oxidation catalyst once in presence of water because of the photogeneration of OH radicals by neutralization of OHâ surface groups by positive holes. Many toxic inorganic ions are oxidized in their harmless upper oxidized state. The total degradation of organic pollutants (pesticides, herbicides, insecticides, fungicides, dyes, etc. ...) is the main field of water photocatalytic decontamination. The UVA solar spectrum can de advantageously used as demonstrated by many campaigns performed in the solar pilot plant at the âPlataforma Solar de Almeriaâ (Spain)